The question the proposed study addresses is: How is energy expenditure related to dietary intake and somatic growth, considering infant characteristics, infant activity, and food intolerance, and controlling feeding practice (caloric density of the diet and ad libitum or prescribed feeding regimen)? The specific aims of the study are as follows: (1) To determine the energy expenditure in the three groups of preterm infants during levels of activity, ranging from quiet sleep to vigorous activity with crying and including feeding; and (2) To determine the contribution energy expenditure makes to the dietary intake and somatic growth of these infants, taking infant characteristics (birthweight, gender, ethnicity, severity of lung disease, and postconceptional age at the time of energy expenditure assessment), infant activity, and food intolerance into account and controlling the caloric density of the diet and the feeding regimen. A 2 x 2 experimental design with random assignment of infants to either ad libitum or prescribed feeding, and, within the feeding practice, to 20- or 24-calorie/ounce feeding substance will be used. Twelve infants will be randomly assigned to each of the four feeding practices. All Infants admitted to the Neonatal Intensive Care Unit (NICU) born at less than 35 weeks gestation with weight that is appropriate for gestational age and who are exclusively formula fed or fed at least 80 percent breast milk will be eligible for this study. Study infants must not have congenital or medical problems interfering with nutritional status. Infants may have recovered from Respiratory Distress Syndrome (RDS) or have mild Bronchopulmonary Dysplasia (BPD). The four study arms will be balanced with respect to the following possible covariates: (a) birthweight, (b) gender, (c) ethnicity, (d) respiratory status (RDS, BPD, or none), and (e) post-conceptional age at the time of energy expenditure assessment. Energy expenditure will be observed with indirect respiratory calorimetry on the first and second day the infant is completely nipple fed at ninety minutes post-prandial for a ninety-minute period each day and with mean heart rate during feeding. The infant's six-hourly dietary intake and daily weight will be the basis for the following measures over the 72-hour study period: (1) mean volume in cc/kg/day of intake; (2) mean kcalories/kg/six-hours of intake; and (3) mean g protein/kg/six-hours of intake. Mean grams weight gain/six-hour period (rate of weight gain) divided by the weight that day will be the measure of somatic growth. Dietary intake outcomes will be analyzed using iterative generalized least-squares regression against the following covariates: (a) NICU location, (b) food intolerance, (c) energy expenditure, (d) severity of lung disease, and (e) feeding practice. The weighted rate of weight gain will be regressed against dietary intake and the other previously listed covariates. The interaction of the two feeding practice factors (feeding regimen and caloric density) will be examined.